Gaétan Daigle

Scales of substratum heterogeneity, structural complexity, and the early establishment of a marine epibenthic community

Abstract This study examines experimentally, in situ, the influence of four scales of heterogeneity (0, 1, 10, 100 mm) and the hierarchical combination of those scales of heterogeneity (0 + 1 + 10, 0 + 1 + 100, 0 + 10 + 100, 0+1 + 10+100 mm) on the early establishment of a sessile marine littoral epibenthic community. The study was carried out in St. Andrews, New Brunswick (Canada) from July to October 1989. Flat panels without and with crevices of various depths (1, 10, 100 mm) alternating with adjoining flat areas were moored according to a random design in the surface 2 m in the sublittoral zone. Species diversity and total percent cover increased from August to October. However, the ANOVA showed no significant effect of substratum heterogeneity nor complexity (the hierarchical combination of various scales of heterogeneity) on overall diversity (H′) in August and October nor on percent cover on the whole panels in August. However, one type of complex panel out of eight types tested did show a significantly lower percent cover than the others in October. Small scale patterns of distributions varied significantly among the different types of surfaces within each panel, however. In general, percent cover of organisms were greater in 1 mm crevices in August and in 10 mm crevices 2 months later, than on other types of surfaces. Local hydrodynamics and active larval choice may be involved. Our study shows that both substratum heterogeneity and complexity did not influence much the overall northern sessile community characteristics (diversity and abundance), but structured strongly the small scale distribution of its species during the early phases of development.

Spatial and Temporal Dynamics of Mass Mortalities in Oysters Is Influenced by Energetic Reserves and Food Quality

Although spatial studies of diseases on land have a long history, far fewer have been made on aquatic diseases. Here, we present the first large-scale, high-resolution spatial and temporal representation of a mass mortality phenomenon cause by the Ostreid herpesvirus (OsHV-1) that has affected oysters (Crassostrea gigas) every year since 2008, in relation to their energetic reserves and the quality of their food. Disease mortality was investigated in healthy oysters deployed at 106 locations in the Thau Mediterranean lagoon before the start of the epizootic in spring 2011. We found that disease mortality of oysters showed strong spatial dependence clearly reflecting the epizootic process of local transmission. Disease initiated inside oyster farms spread rapidly beyond these areas. Local differences in energetic condition of oysters, partly driven by variation in food quality, played a significant role in the spatial and temporal dynamics of disease mortality. In particular, the relative contribution of diatoms to the diet of oysters was positively correlated with their energetic reserves, which in turn decreased the risk of disease mortality.

Toward management guidelines for the soybean aphid in Quebec. I. Feeding damage in relationship to seasonality of infestation and incidence of native predators

A study was conducted in 2004 and 2005 to test the hypotheses that the severity of damage caused by the soybean aphid, Aphis glycines (Hemiptera: Aphididae), is minimized by the activity of predators and declines with the maturity of soybeans, Glycine max (L.) Merr. (Fabaceae), at the time of infestation. In caged subplots where predators were excluded, aphids attained a high density following experimental infestation of soybeans, resulting in severe reductions of yield, particularly when plants were infested early in the season. A guild of generalist predators consisting predominantly of ladybird beetles colonized plants in uncaged subplots, resulting in a low rate of population growth following infestation of soybeans with aphids and a relatively weak impact on the soybean yield. The soybean yield declined as the density of aphids (number per plant), and the maturity of soybeans at the time of infestation, increased. Our results suggest that A. glycines represents an occasional pest of soybean in Quebec...

A variance-covariance structure to take into account repeated measurements and heteroscedasticity in growth modeling

This study proposes a within-subject variance-covariance (VC) structure to take into account repeated measurements and heteroscedasticity in a context of growth modeling. The VC structure integrates a variance function and a continuous autoregressive covariance structure. It was tested on a nonlinear growth model parameterized with data from permanent sample plots. Using a stand-level approach, basal area growth was independently modeled for red spruce (Picea rubens Sarg.) and balsam fir [Abies balsamea (L.) Mill.] in mixed stands. For both species, the implementation of the VC structure significantly improved the maximum likelihood of the model. In both cases, it efficiently accounted for heteroscedasticity and autocorrelation, since the normalized residuals no longer exhibited departures from the assumptions of independent error terms with homogeneous variances. Moreover, compared with traditional nonlinear least squares (NLS) models, models parameterized with this VC structure may generate more accurate predictions when prior information is available. This case study demonstrates that the implementation of a VC structure may provide parameter estimates that are consistent with asymptotically unbiased variances in a context of nonlinear growth modeling using a stand-level approach. Since the variances are no longer biased, the hypothesis tests performed on the estimates are valid when the number of observations is large.

Statistical Methods for Aerial Surveys Using the Double-Count Technique to Correct Visibility Bias

In double-count surveys two observers search the sampled area for the species of interest. The presence of the two observers permits one to calculate a survey-specific correction for visibility bias. This correction factor can be negatively biased, because animals difficult to see for one observer are also difficult to see for the other. This paper investigates methods for reducing this bias. One solution is to classify, during the survey, the animals seen according to variables potentially influencing their visibility. Statistical methods are proposed to evaluate the efficiency of such a classification. Another proposal to reduce the bias of double-count estimators is to use parcelspecific correction factors. Variance estimators for double-count estimators of population totals are proposed; three variance components are identified. The proposed techniques are illustrated using data collected in a double-count deer survey carried out on Anticosti Island in the Gulf of St. Lawrence.

Fluctuating asymmetry and mortality in cultured oysters (Crassostrea gigas) in Marennes-Oleron basin

Japanese cupped oysters suffer summer mortality in many culture sites along the French Atlantic coast. To ascertain whether mortality might be associated with morphological features of the shells, we estimated fluctuating asymmetry (FA) of hatchery/nursery-produced oysters kept in culture bags in the intertidal. FA is defined as random variations from a perfect symmetry plane (bilateral or radial) in some morphological trait. FA increases with disruption of homeostasy during ontogeny and has both genetic and environmental determinants. We found significant differences in FA between dead and live oysters in two of our three study groups. Therefore, lack of developmental stability may be involved in the summer mortality syndrome of oysters. We also found indirect evidence that once acquired, FA patterns of shells are retained at least partially. Since FA likely appears early during ontogeny and is recorded in the shells, we hypothesise that it might be useful for forecasting the ability of spat to resist environmental stress.

Growth, survival and fluctuating asymmetry of Iceland scallops in a test of density-dependent growth in a natural bed

We report an experiment designed to test for overpopulation in an Iceland scallop, Chlamys islandica (O.F. Muller), slow-growing bed. Scallops were installed in pearl nets at the site of the scallop bed and in a nearby unpopulated site, near the bottom and far from the bottom. With this set- up, all groups, except that one near the bottom inside the bed, were free from potential overpopulation effects. In addition, there were two stocking densities to test for containment effects. Shell growth was higher at 2.0 m above the bottom, outside the scallop bed, than at the three other site and height combinations, which were not significantly different from each other. There was a borderline effect of site on soft tissue growth, which was probably slower at the donor site. We conclude that there was no evidence of overpopulation in the scallop bed. On both sites, soft tissue growth was slowest near the bottom, irrespective of group size, intermediate at 2.0 m height, high population density, and fastest at 2.0 m height, low population density. Survivorship was lower near the bottom, but was independent of site and of group size. Available data showed no sustained vertical patterns in phytoplankton concentration in the water column. These results suggest that food depletion occurred in the pearl nets, but that density-independent factors dominated density- dependent factors in the pearl nets near the bottom. Likely explanations involve the interaction between containment effects and current speed variations in the benthic boundary layer. Survivorship decreased with fluctuating asymmetry of the ears of the shells. Fluctuating asymmetry, therefore, may provide a means of including individual effects in growth experiments and

Environmental factors as predictors of epibenthic assemblage biomass in the St. Lawrence system

The distribution of epibenthic invertebrate biomass in relation to environmental factors was examined in the St. Lawrence system. Biomass estimates for epibenthos sampled yearly for 9 years on 102 suspended collectors (navigation buoys), were related to environmental data from the literature (surface water temperature, water salinity, water transparency, current velocity, chlorophyll a and primary production) using a weighted multiple linear regression analysis. Regression models were generated for total biomass and the biomass of the single dominant sessile species: Mytilus edulis, Semibalanus balanoides, Balanus crenatus, Obelia longissima and Hiatella arctica. Water temperature and water transparency, as well as some biogeographic groups of buoys represented by dummy variables, collectively explained 90.6% of the variance in total biomass. Water temperature, water transparency, biogeographic groups and, to a lesser degree, primary production, were the variables having a significant influence on the biomass of individual species. The lognormal weighted multiple regression model explained up to 84.5% of the variance in M. edulis biomass data and 67.9, 70.0, 71.6 and 38.9%, respectively, of the variance in S. balanoides, O. longissima, B. crenatus and H. arctica biomass data. The need to consider simultaneous biological and environmental sampling at the relevant temporal and spatial scales to model large marine coastal ecosystems is discussed. 2003 Elsevier Science B.V. All rights reserved.

Otolith Microstructure during the Early Life-History Stages of Brown Trout: Validation and Interpretation

Abstract We examined the extent to which otolith microstructure provides an accurate estimate of age, growth, and early life history transitions during the period between hatching and 1 week after emergence in Brown Trout Salmo trutta exposed to natural variations in ambient water temperature. All fry analyzed possessed a prominent check on the observed date of hatching. After hatching, daily growth increments were visible on sagittal otoliths. There was no evidence for the formation of an emergence check mark and no statistically significant evidence that emergence and daily temperature fluctuations interacted to form check marks. However, daily temperature fluctuations may influence the formation of check marks, largely based on an observed increase in the proportion of fish possessing checks on the days following the two largest temperature fluctuations observed during the experiment. There was no evidence that feeding or stressing emergent fish contributed to the formation of an emergence check mark. ...

Shelter use and behaviour of juvenile Spotted Wolffish (Anarhichas minor) in an experimental context

Laboratory experiments were conducted to assess shelter use and behaviour of juvenile Spotted Wolffish, Anarhichas minor, using time-lapse video. Information about the behaviour of Spotted Wolffish and its habitat utilization is sparse due to the great depth at which this species lives. Four experiments were conducted using one or two fish per tank, with and without a shelter. The positions and movements of fish in the tanks and interactions between fish were monitored over 24-h periods. In experiments in which no shelter was available, Spotted Wolffish spent very little time exploring the water column. They did not show fidelity to a particular area on the bottom of the tank and exhibited few signs of aggressiveness to each other. When a shelter was made available, fish spent most of their time in the shelter or close to the shelter. The presence of a single shelter did not enhance aggressive behaviours in fish kept in pairs. When day and night observations were compared, no clear diel pattern emerged. The Canadian recovery plan for Spotted Wolffish calls for research aimed at describing their use of demersal habitats. The present study suggests that shelter availability might be an important feature of the habitat requirements of juvenile Spotted Wolffish.